<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>promise-ajax</title>
</head>
<body>
    <button id="btn">ajax</button>
    <script>
        btn.onclick = function(){
            let p = new Promise((resolve, reject)=>{
                const xhr = new XMLHttpRequest();
                xhr.open('get','http://127.0.0.1/json');
                xhr.responseType = 'json';
                xhr.send();
                xhr.onreadystatechange = function(){
                    if(xhr.readyState === 4){
                        if(xhr.status >=200 && xhr.status < 300){
                            resolve(xhr.response);
                        }else reject('请求失败');
                    }
                }
            })
            p.then(value=>{
                console.log(value);
            })
        }
    </script>
</body>
</html>